In the context of fuel tanks, “inerting” may refer to the process of making ullage in a fuel tank non-flammable. The Federal Aviation Administration (FAA) reconsidered aircraft fuel tank safety after a series of fuel tank explosions between 1990 and 2001. The National Transport Safety Board (NTSB) added “Explosive Mixture in Fuel tanks in Transport Category Aircrafts” as the number one item in its 1997 “Most Wanted” Transportation Safety list.
Some known fuel tanks have a region containing liquid fuel and an ullage region that often contains evaporated fuel (i.e., fuel vapor). With the presence of air, the mixture may exhibit a fuel-air ratio within the ullage and ignition may occur when fuel-air ratio in the ullage lies within a certain range. The lower flammability limit is defined as a threshold below which the fuel-air ratio is too lean to ignite. Similarly, the upper flammability limit is defined as the threshold above which the fuel-air ratio is too rich to ignite. The fuel-air ratios between the lower flammability limit and the upper flammability limit are termed flammable.
At temperatures less than 100° F. at sea level, the ullage fuel-air ratio for Jet A fuel generally lies below the lower flammability limit. However, known conditions exist that may result in exceeding the lower flammability limit. One example includes a rapid reduction in tank ullage pressure after take off, such as when the aircraft reaches a high altitude in a short time before fuel-tank temperature substantially decreases.
FAA regulations require that new and in-service transport aircraft include systems for enhancing the safety of aircraft fuel tanks. One known system uses an inert gas, such as nitrogen, to reduce the oxygen concentration in ullage. Such a system is expensive, complex, and increases weight of the aircraft. Another known system cools ullage mixture to condense out fuel vapors and to maintain the fuel tank ullage at a relatively low fuel-air ratio, significantly lower than the flammability limit. It is possible to improve methods and systems for reducing flammability of fuel tanks.